Pipe Volume Calculator
How Much Water Is Sitting Inside That Pipe Right Now
A pipe is a cylinder, and the water inside it at any moment is simply that cylinder's volume. This matters more often than it might seem — from calculating how much water is wasted waiting for hot water to arrive, to estimating drain-down volume before cutting into a line. This calculator handles both imperial (inches and feet) and metric (centimeters and meters) inputs.
The Formula
Imperial: gallons = V(ft³) × 7.48052
Metric: liters = V(m³) × 1000
Radius is derived from the entered diameter (half of diameter, converted to feet or meters as needed) before the cylinder volume formula is applied.
Where This Calculation Matters
- Hot water wait-time and waste — the volume of cold water that must clear a supply line before hot water arrives at a fixture is exactly this pipe volume.
- Drain-down before repairs — estimating how much water will empty out of a line before cutting or removing a section.
- System water content — radiant heating and hydronic systems need total loop water volume for expansion tank and glycol calculations.
- Chemical dosing — treatment or flushing chemicals in a pipe run are often dosed relative to the water volume they need to treat.
Volume per Linear Foot by Diameter
| Diameter | Gallons per foot |
|---|---|
| 0.5 in | 0.0102 gal/ft |
| 0.75 in | 0.0229 gal/ft |
| 1 in | 0.0408 gal/ft |
| 1.5 in | 0.0918 gal/ft |
| 2 in | 0.1632 gal/ft |
| 3 in | 0.3672 gal/ft |
| 4 in | 0.6528 gal/ft |
| 6 in | 1.4688 gal/ft |
Because volume scales with the square of diameter, a 4 in pipe holds roughly sixteen times as much water per foot as a 1 in pipe, not just four times.
How to Use This Calculator
- Choose a unit system: Imperial (in / ft) or Metric (cm / m).
- Enter the pipe diameter.
- Enter the pipe length.
- Select Calculate to get the water volume in gallons/cubic feet (imperial) or liters/cubic meters (metric).
Related Calculations
To find the flow rate moving through that same pipe, use the Pipe Flow Calculator. For the correct fall needed if this run is a drain line, see the Drain Slope Calculator.